The intensive research currently conducted on substitutes for chlorofluorocarbons (CFCs) is directed, inter alia, towards the synthesis of hydrohaloalkanes. Some steps of this synthesis may be carried out by fluorination with hydrofluoric acid by heterogeneous gas phase catalysis.
Very numerous metal compounds (for example chromium, cobalt, nickel, iron, copper, manganese, and the like) have a catalytic effect for these fluorination reactions. The catalysts proposed in the literature are either mass catalysts or supported catalysts, the support being mainly carbon or alumina (partially convened to AlF.sub.3 after fluorination).
Amongst this second category, a large variety of metal compounds are found and there are numerous patents describing fluorination processes based on this type of catalyst. Thus, mention may be made of the patents U.S. Pat. Nos. 2,744,147 and 2,744,148, which describe the fluorination of a haloalkane on a catalyst based on a metal (chromium, cobalt or nickel for the patent U.S. Pat. No. 2,744,147 and chromium, cobalt, nickel, copper or palladium for the patent U.S. Pat. No. 2,744,148) supported on alumina.
More recently, the patent EP 0 366 797 describes a fluorination process using a catalyst consisting of at least one metal (nickel, cobalt, iron, manganese, chromium, copper and silver) fluoride supported on an alumina having a significant mesoporosity.
For all of these catalysts, the support imparts a certain solidity to them. However, as the amount of active material is smaller than in a mass catalyst, the catalytic activity may be affected. Moreover, these low contents of non-precious metals do not enable their recovery from spent catalysts to be envisaged in an economic manner.
The mass fluorination catalysts are mainly based on chromium and the starting materials used for their development are very varied (salts, oxides, halides and the like).
Thus, the patents U.S. Pat. No. 4,912,270 and EP 0 313 061 claim fluorination processes using catalysts based on chromium oxide which are obtained, respectively, by reduction of chromium trioxide by means of an alcohol and by pyrolysis of ammonium bichromate.
The patent FR 2 135 473 describes the preparation of a catalyst containing chromium and nickel and its use in the fluorination of functional perhalogenated compounds. This catalyst is obtained by thermal decomposition of organic chromium and nickel salts. The nickel contents remain low since the Ni/Ni+Cr atomic ratio is still less than 0.1.
The patent publications JP 2-172932/90 and 2-172933/90 describe, respectively, the fluorination of 1,1-difluoro-1,2,2-trichloroethane (F122) and of 1-chloro-2,2,2-trifluoroethane (F133a) on a chromium catalyst to which a doping metal has been added which enables the reaction temperature to be lowered while maintaining a significant activity and therefore improves the life of the catalyst by restricting the crystallization of the chromium. The chromium/nickel combination is not illustrated by the examples in these publications.
The patent FR 2 501 062 describes the preparation of a mass chromium oxide in the form of microspheres having a diameter of between 0.1 and 3 mm. This catalyst is obtained by gelling a chromium hydroxide sol in a solvent which is immiscible or partially miscible with water. The product obtained is highly solid and is particularly suitable for fluorination reactions in a fluidized bed.
The disadvantage of catalysts based on chromium oxide is their low resistance at high temperature (350.degree.-500.degree. C.) to the crystallization which contributes to reducing their life.
Moreover, these chromium-based catalysts promote the oxidation of hydrochloric acid by the oxygen dissolved in the reagents or deliberately introduced. Water and chlorine form by a Deacon reaction (Chemical Week 1987, 24 Jun., page 18) and these in turn react with the organic compounds, which leads to the formation of non-valorizable by-products and, consequently, to a lowering of the selectivities.
It has now been found that the addition of a nickel compound to a chromium derivative in order to form a sol of chromium and nickel hydroxides makes it possible, while retaining the advantages of a mass catalyst, not only to prolong the life of the catalyst by improving the resistance of the chromium-based compound to crystallization but also to improve the selectivities in gas phase fluorination reactions owing to a partial inhibition of the oxidation of hydrochloric acid in the presence of chromium.